Toughening mechanisms in discontinuous glass/polypropylene composites

Eunethia Denise Williams

doi:10.17918/00008925

There are five energy absorbing mechanisms for discontinuous fiber reinforced composites. These include fiber debonding, fiber pullout/bridging, fiber breakage, matrix cracking and matrix deformation. Because of the typically random orientation of fibers in these systems, it has been difficult to develop an experimental approach to determine which of these contributions dominate the measured in-plane toughness values. To address this issue, a specimen has been developed of one pultruded sheet of polypropylene-glass longitudinally centered in a compression molded polypropylene bar. Experimentally, interface strength was varied from weak to intermediate to strong adhesion through the addition of different sizings and maleic anhydride grafted polypropylene. The pullout contribution was determined through varying fiber length. Matrix contributions were determined by changing the molecular weight of the matrix and by changing the crystal structure from predominantly the alpha form to the beta form. Experiments of Izod testing have shown that impact strength is maximized in specimens with a weak interface at an optimum fiber length corresponding to the fiber length at which pullout predominates. In specimens with an intermediate strength interface, impact strength showed a maximum but was reduced. In specimens with a strong interface, impact strength was the lowest and fairly level. Experiments of tensile testing also showed maximum toughness in specimens with a weak interface with an optimum fiber length where pull out predominates. In specimens with a strong interface, tensile toughness was reduced. Examination of these specimens suggests that the impact strength is maximized by the density via the crystallinity of the resin. The density influences the volume contraction of the matrix around the fibers during cooling. This volume contraction creates a radial stress on the fibers that affects the frictional shear stress against which the fibers are pulled out during fracture. A higher density translates into a higher frictional stress and therefore a higher toughness. It was found that neither the crystal structure nor the matrix toughness altered the controlling effect of the matrix density on the impact strength. On the other hand, the tensile toughness was maximized by the yield stress of the matrix for the weak interface.

Toughening mechanisms in discontinuous glass/polypropylene composites

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Toughening mechanisms in discontinuous glass/polypropylene composites

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